In the distant future, a couple is trapped in traffic and something apparently attacks their vehicle. They try to summon the police but the rear of their vehicle explodes while they scream in terror.
The Doctor decides to take Martha to the distant future, to New York on New Earth in the year 5,000,000,003. They end up in a dismal alleyway in the middle of a rainstorm, and find their way to a street filled with stores that sell all manner of mood-altering drugs. A woman comes to by a Forget dose and mentions that her parents were killed on the motorway. The Doctor takes an interest but she takes the drug and doesn’t remember anything useful. A couple then grab Martha and duck through a doorway, locking it behind them. By the time the Doctor catches up to them, they escape a hover vehicle. The Doctor gains access to the motorway and finds himself looking at a vast underground causeway filled with emission fumes and tens of thousands of hovercars.
Elsewhere, the Face of Boe senses that the Doctor has arrived and sends an elderly cat-nun to find him. She picks up a gun and departs.
Martha wakes up and confronts the couple, Milo and Jean. They explain that they needed three people to get access to the Fast Lane on the lowest level of the motorway, and intended her no harm. However, they quickly note that they’ve been driving for months and have moved only a few miles, so Martha may be with them for quite a while.
A cat-man, Thomas, and his wife Valerie take pity on the Doctor and let him in, then explain that they’ve been driving for years, and their children were born while they were on the motorway. They move a few yards and the Doctor is forced to stay aboard. He tries to call the police but is put on hold. They call an elderly married couple who “car spot” and manage to identify Martha’s vehicle, but it is further below heading for the fast lane and they have no way to get there. Valerie refuses, warning that it’s dangerous.
Further below, Martha and her newfound friends hear strange growling noises but Milo dismisses it as the sound of the air vents. Jean describes it as legendary monsters rumored to live in the lowest levels. Meanwhile, the Doctor ascertains that no one has ever seen a police vehicle… or any other kind of service vehicle. Realizing he’s getting nowhere, the Doctor starts dropping down from car to car to get to the fast lane.
Martha’s car comes under attack by unseen creatures and another driver is attacked as well while trying to warn them. Martha tells Milo to drive for their lives.
The Doctor has arrived at the last line above the fast lane, where there is a thousand-foot drop into impenetrable mist. He taps into the systems and summons up a breeze to reveal giant crab creatures below: the Macra. The Doctor has encountered them in the past, and recognizes them as a race of conquerors who feed on gas and once used an enslaved human colony to mine “food” for them.
He’s interrupted when the cat-nun bursts in, but is friendly and introduces herself as a now-elderly Novice Hame, who the Doctor met on his last visit. She says the Doctor must come with him and when he refuses, she grabs him and teleports him up to the Overcity. They appear in the city’s Senate Chamber, where everyone has been dead for 24 years. Hame explains that everyone was taking a new drug, Bliss, and it mutated into a lethal form that killed everyone in the Overcity. As a last-ditch measure, they sealed off the motorway, trapping but keeping alive everyone who was driving at the time. The galaxy then quarantined the planet for 100 years. Hame also introduces the Face of Boe, who has been using his lifeforce to maintain the motorway all this time.
Milo has shut down the vehicle to avoid the Macra but Martha insists the Doctor will save them. They drive through the horde of creatures, barely avoiding their claws, and the Doctor picks them up on the sensors. He tries to tap into the Overcity power systems but has no luck until the Face of Boe lends the last of its life force sot that the Doctor can open the vents above the motorway. He gets on the broadcast system and tells everyone to get out. Martha and her friends escape just in the nick of time.
Martha goes to the Senate Building where she finds the doctor… and the Face of Boe, dying. Before the alien dies, he tells the doctor that he is not alone. The Doctor and Martha head back to the TARDIS where Martha demands answers, since the Doctor hasn’t told her anything so far. He finally relents and explains that he is the Time Lord, the last of his kind when the others died in a great Time War against the Daleks. As they talk, overhead the newly-returned residents of New York resettle their city.